I tried most things I could think of, re-installing firefox, re-installing flash, running firefox in safe mode (with all add ons disabled) and manually uninstalling suspect add-ons in normal mode, none of which fixed it.
One thing that did make it work is swithcing to inprivate browsing (which as I understand it simulates using a different profile). Even now, I have all the same add ons installed in my new profile and it works perfectly, if I switch back to my old profile, just a black screen.
Very odd indeed.